1. Field of the Invention
This invention relates to a phase-locked transmission system and, more particularly, to a simulcast transmission system having a plurality of phase-locked remote transmitters.
2. Description of the Prior Art
Simultaneous broadcast, or simulcast transmission systems, typically have a plurality of remote transmitters that simultaneously broadcast identical audio or information signals at substantially the same carrier frequency. By using a number of geographically diverse transmitters, a simulcast system provides maximum signal coverage to topographically irregular geographical areas, such as mountainous regions, where unobstructed radio coverage is not possible.
The performance of simulcast transmission systems is greatly degraded if the transmitters assigned the same frequency are not operating on substantially the exact same frequency. Frequency offset between transmitters causes audible interference in the form of beat noise. This problem has been typically solved by utilizing very high stability oscillators in each of the transmitters. However, such an approach is expensive and has a number of problems including temperature susceptibility and frequency drift caused by aging of the high stability oscillator necessitating costly periodic maintenance.
The problem of frequency drift is more critical for transmission of data signals than voice signals. Data signals are highly susceptible to beat noise which alters data bits rendering entire data sequences totally incomprehensible. The falsing of data signals may be remedied to some degree by the use of complex error-correction codes requiring complex and expensive coding devices at each station of a simulcast transmission system.
For the foregoing and other shortcomings and problems, there has been a long felt need for an improved simulcast transmission system having transmitters at remote stations with more accurately controlled frequencies.